1. Field of the Invention
The present invention relates generally to polymer films. More particularly, the invention relates to polymer films microencapsulated by means of a multidirectionally stretch-crazing process and to a method of manufacturing such films.
2. Description of Related Art
For better understanding of the present invention, it would be expedient to define some terms used in the present patent application.
Encapsulation is a technological process in which small amounts of substances known as cores are enclosed within another material known as a matrix material. The types of substances that may be encapsulated include solids, liquids, gases, as well as mixtures, suspensions, emulsions, and solutions of the above.
When the core of an encapsulated substance is around 300 .mu.m or smaller, such encapsulation is referred to as microencapsulation. Our invention applies to microencapsulation of substances with crazes having in an opened state a width from from 50 .ANG. to 100 .mu.m in width and many times of that in length.
Encapsulating walls of microcapsules may have a thickness from a fraction of a micron to tens of microns. The walls can be either rigid or elastic and comprise one or more layers. These encapsulating walls are usually made from high molecular compositions of animal or plant origin, such as proteins and synthetic polymers, and also from paraffin and stearin.
A great many microencapsulation techniques are known. Among them, the most common techniques are described in Encyclopedia of Chemical Technology, Vol. 13, 2nd Ed., N.Y., 1967, p. 436. All these techniques are based on processes of film formation in heterogeneous systems on interfaces between liquid and liquid, liquid and solid, gas and liquid, and/or gas and solid.
In terms of the film-formation principle, all known methods of microencapsulation can be roughly divided into the following three groups.
1) Formation of films from film-forming solutions with controlled rate of film formation. The mechanism of this technique is based on the use of solutions of a film-forming substance in an organic solvent or water. The microencapsulation is accomplished by isolating a phase enriched with the film-forming substance from multiple-component systems that constitute a dispersion of the substance to be encapsulated in a predetermined solution. This can be achieved by varying the temperature of the solution, its pH, or by evaporating a portion of the solvent. As a result, microscopic drops of the phase enriched with the film-forming substance are deposited on the surface of the material to be encapsulated thus forming a core covered with a continuous shell of the coating material.
Parameters of the aforementioned process are selected based on known phase diagrams of the multiple-component systems. Microencapsulation by that method, e.g., in a fluidized-bed layer, is used mainly for encapsulation of pharmaceutical substances where the duration of the process in some cases takes 5 to 20 hours. Another disadvantage of microencapsulation from a solution is that this process requires many technological steps, i.e., preparation of solution, 4 application of the solution, evaporation, hardening, and other steps, each requiring strict control of the operation conditions.
Other limiting requirements of the above process are non-solubility of the core material in the coating material and vice verse, a greater surface tension on the core than in the phase enriched with the film-forming material, and a lower surface tension than on interface of these phases, etc.
Other disadvantages of the encapsulation from the solution are low stability of the properties of the coating material in time, low storage stability, etc.
2) Film formation from a melt. This process is based on encapsulation of drops of a liquid core material. One example of this process is passing drops of a core material with a high speed through a liquid of a coating material, so that when the drop has passed through the liquid coating material, a portion of the latter remains on the surface of the drop and solidifies forming a shell. The aforementioned method is used for encapsulation by saturated solutions of salts, water, and glycerin with various thermoplastic materials. A disadvantage of the capsules produced by the above method is a risk of migration of low-molecular-weight substances from the coating layer to the core.
3) A third well-known method of microencapsulation is formation of a core-coating film as a result of polycondensation or polymerization.
To perform polycondensation on the interface between the core and coating, one of monomers is dissolved in an organic solvent and another in water that contains a small amount of a catalyst. A core material is introduced into one of the phases. In this process, the organic phase should neither dissolve the polymer formed in the process nor mix with water. With the introduction of one of the phases into another, e.g., through a dispersing nozzle, polycondensation immediately begins on the surfaces of particles that contain the core material. This process is accompanied by separation of microcapsules which float up or falls down, depending on their specific gravity.
When it is necessary to obtain microcapsules with dimensions from fractions of micron to several microns, polymerization is performed on the interface with a gaseous phase. Methods used for the preparation of aerosols may be utilized for encapsulation of the core material in an inert gas environment by combining the inert gas with vapors of a monomer capable of catalytic polymerization. [See U.S. Pat. No. 4,604,444 issued in 1986 to Donnadieu.]
Microencapsulation on the basis of polycondensation and polymerization to a great extent depends on the temperature of the process. Another serious disadvantages of this process is a risk of incomplete polymerization or polycondensation of the monomers. Since in many cases such monomers are toxic, their penetration into the core material may lead to undesired consequences.
All three methods described above relate to cases where a target core material is coated with a continuous capsule material, i.e., the coating has to be formed on the surface of material to be encapsulated. Such a coating process itself is highly substance-dependent: small variations in the material of the core and/or the material of the coating substance may require considerable alternations of the technological parameters, or even switching to another process of coating altogether. The aforementioned known processes are expensive as they require the use of complicated equipment, are difficult to control, and require a skilled labor. Furthermore, many of the procedures used to microencapsulate substances, such as drugs, do not adapt well to a large-scale commercial process either from mechanical or economical point of view. In other words, the disadvantages described above result from the nature of the technological processes used for encapsulation and therefore are unavoidable.
Attempts have been made to eliminate the disadvantages inherent in methods of encapsulation by coating the core material with a continuous capsule, in other words, to incorporate the material to be encapsulated into another ready-made matrix material with preselected properties.
For example, USSR Inventor's Certificate Application No. 2774421 filed on Jan. 6, 1979 by one of the authors of the present patent application describes a method of manufacturing long-lasting decorative films for the building industry by encapsulating a dye into a polymer film. This method is based on a uniaxial stretching of a film immersed into a coloring medium, whereby discrete cracks or crazes are formed and filled with the coloring medium. When stretching is discontinued, the crazes are closed leaving some of the coloring material encapsulated inside the film.
Crazes are minute cracks which are formed in the surface of a material such as polymer due to different properties on the surface and inside the material.
A disadvantage of this method consists in that the amount of encapsulated material is limited so that this method leaves most of the cavity-creating potential untapped. Therefore this method did not find practical application, even for decoration purposes.
It can be concluded that all of the currently applied methods of microencapsulation have the following three shortcomings inherent in the their technology. First, they are highly substance-specific. A slight change in surface properties, solubility, or some other characteristic of the encapsulated substance or encapsulating material, may cause the failure of the process. Second, the encapsulating is accompanied by chemical reactions, or a sudden change in temperatures or pressure. There is always potential for forming unwanted byproducts. Third, many of these processes are not suitable for mass production. The method described in the above mentioned Inventor's Certificate, although it does not have the shortcomings of the currently applied microencapsulation methods, is too ineffective to become a viable commercial technology even in its narrowly stated purpose, i.e., making decorative films.
Along with the microencapsulated products described above in the form of separate capsules consisting of a core covered with a coating, there is another group of microencapsulated products in the form of sheets or films that contain microcapsules.
For example, such films may be used as self-adhesive films that contain microcapsules with an adhesive substance applied onto the surface of the film. When a pressure is applied to the film, the capsules break, and the adhesive substance is released. [See Chem. Eng. Progress, 62, nO. 4, 87, 1966].
The manufacturing of such a product involves, apart from the production of the film, two more labor-consuming and expensive steps, i.e., microencapsulation and attachment of microcapsules to the film. Furthermore, the micro-encapsulation used in these processes have all the shortcomings inherent in conventional microencapsulation processes described earlier. Another problem of the method of manufacturing microencapsulated adhesive films consists in that the microcapsules have to uniformly distributed over the surface of the film.
Another example of a sheet-like microencapsulated product is a pressure-sensitive copying paper that contains microcapsules of a liquid pigment attached to one side of the paper. When a pressure is applied to the other side of the paper which is placed onto copy paper, the microcapsules which experience pressure are broken, the pigment is released, and transferred to copy paper. The pressure-sensitive copying paper possesses the same disadvantages as those inherent in the microencapsulated self-adhesive films.
Other examples of microencapsulated sheets and films are photosensitive paper (sensitive to UV rays) and a magnetic recording tape. Such products also require two additional steps, apart from the manufacturing of the film, i.e., the preparation of microcapsules and their attachment to the substrate. It is essential to note with regard to these specific products that their main characteristics such as resolution capacity and quality of the image depend exclusively on the uniformity and density of distribution of the microcapsules. These requirements further contributes to complexity and cost of manufacturing of such products.
A good example of products that contain films coated with microcapsules are dry cells used as a source of electric power. An essential part of such a cell is a polymer fiim coated on one side with silver peroxide (used as a negative electrode) and zinc on the other side of the film (as a positive electrode). In addition, the surface of the film is coated with capsules containing a KOH solution. The films are stored in rolls and are in contact with pressure rolls and current-collecting rolls. When it is necessary to activate the cell and to induce a voltage on the cell's terminals, the film is squeezed between the pressure rolls, the alkaline solution is released from the capsules, and an electrochemical reaction that occurs between the materials of both electrodes in the alkaline solution generates electric energy. [See previous reference.]
Microencapsulated films find various applications in medicine and pharmacology. For example, films are produced with microcapsules containing liquid crystals of certain fatty acid ethers and cholesterol which change their color under the effect of temperature. Such films are used as thermometers by applying the color-temperature-sensitive film on the surface of the patient's body.
Thus is can be concluded that all known film or sheet-like microencapsulated products are expensive to manufacture, involve many technological manufacturing steps, require strict control of the distribution and density of the microcapsules, and complicated and expensive method of controlling the amounts of substances to be microencapsulated. Furthermore, micro encapsulation of the films or sheet-like materials by applying microcapsules onto their surface impairs mechanical and physical characteristics of the initial film.